(1) Field of the Invention
This invention relates to an image sensor, and an imaging apparatus using the image sensor, for use in the medical field, industrial field, nuclear field and so on.
(2) Description of the Related Art
An imaging apparatus that obtains images based on detected light or radiation has a light or radiation detector for detecting light or radiation. Such a detector will be described, taking an X-ray detector for example. An X-ray detector has an X-ray converting layer (semiconductor layer) of the X-ray sensitive type. The X-ray converting layer converts incident X rays into carriers (charge information).
The detector detects the X rays by reading the carriers. Amorphous selenium (a-Se) film, for example, is used as the X-ray converting layer (W. Zhao, et al., “A flat panel detector for digital radiology using active matrix readout of amorphous selenium”, Proc. SPIE Vol. 2708, pp. 523-531, 1996).
In a radiographic operation carried out by irradiating an object under examination with X rays, radiographic images transmitted through the object under examination are projected onto the amorphous selenium film, thereby generating carriers proportional to the densities of the images in the film. The carriers generated in the film are collected by carrier collecting electrodes in a two-dimensional arrangement. After the collection is continued for a predetermined time (called “accumulating time”), the carriers are read outside via thin-film transistors.
Such an X-ray detector has peripheral circuits such as a gate driver circuit for switching the thin film transistors on and off, and an amplifier array circuit for reading the carriers. The driver circuit applies a driving signal to the X-ray detector to drive the X-ray detector. The amplifier array circuit receives the carriers read based on a read signal relating to reading of the carriers. The X-ray detector and these circuits constitute the image sensor.
FIG. 1 shows an outline block diagram of such an amplifier array circuit. As disclosed in Japanese Unexamined Patent Publication No. 2004-23750, and as shown in FIG. 1, the amplifier array circuit includes a charge sensitive amplifier CSA at a first stage, a main amplifier MA at a second stage, and a sampling hold SH at a final stage.
As disclosed in Japanese Unexamined Patent Publication H8-47491, carriers are sometimes read selectively by limiting a predetermined read area to a part of an image field. When, for example, carriers are read from 2,048 by 2,048 pixels in the center of a total field composed of 4,096 by 4,096 pixels, the X-ray detector, gate driver circuit and amplifier array circuit are controlled accordingly.
However, the conventional construction described above encounters the following problem when carriers are read in frame cycles, such that the carriers are read from an image field for one frame, and then from an image field for a next frame, including a partial read mode for selectively reading carriers by limiting each image field to one part. The problem is that a preceding frame imparts an influence on a next frame.